After Starting Pump, How Long Until Discharge Valve Can Open?

First, can we assume that the pump in question is NOT a positive displacement pump?

Centrifugal pumps can be started with the discharge valve wide open, or closed.

Depends on the type of pump, how it is plumbed into the system, what's downstream of the pump, etc.

Left closed too long, the heat build-up inside the volute will cause the seals or pump housing to fail. How much of that 920kW is lost to inefficiencies?

And, it's kW, not KW.

What does the pump manufacturer say about this? Of course, you have talked to them, right? Right?

(Deep sigh, head shake, asks self, why bother?)

It depends on what other pumps and check valves might be present in the system. 90 seconds seems unduly long, but maybe there is a reason?

First why 90 seconds? 10 seconds is MORE than enough

Second, why have the valve closed? Back feed? If so then try a non-return valve. If you are running a centrifugal pump, then as rightly stated in a previous comment, if you leave the valve closed for to long you will damage the pump as the flow of fluid through the pump cools the pump.

Once the pump is running at FULL speed then open the valve.

As a first guesstimate I'd take either 75% of the input electrical power consumed with the pump outlet off, or if that is not known, then maybe 15% if the rated input at max power, and then on the quite reasonable assumption that pretty much 100% of that was going into a heating process that would fairly uniformly heat everything, work out the temperature rise using specific heat (j/kg.deg ) for iron (assuming the pump is iron) and its mass, and the specifjc heat of water and the mass of water in the volume of the pump...and of course a Watt of power is a joule per sec.

The above does not pretend to be accurate, but should it be within a factor of 2 or 3 of the correct answer, and accurate enough to work out if you need to spend more time on the issue.

You also need to be careful with starting your pump delivering fluid production time frame if you are delivering into a empty pipe opening the delivery too quickly can cause the pump to overpump and cause cavitation which can damage your impella ,by opening the delivery as a gradual process this allows pressure in the delivery pipe to rise to your pumps max pressure and self restrict its output to avoid damage. Farmers----Jack of all trades and master of none.

GA

This problem of pumping into an empty pipe at start was dealt with some time ago in one of the topics, but do not remember the date etc.

Therefore, if the pump (Centrifugal !) is started to flow into a long enough empty pipe, there will be a possibility of overloading the pump Motor which will trigger the overload safeties.

If this issue is present, then a gradual opening of the valve will be recommended in a way to keep the back pressure high enough to limit the power demand from the pump, until the system has filled up and normal running is established.This can be done electronically/Automatically or manually.

yes.. good point, but we are assuming the OP is pumping to an empty pipe, he hasn't really stated! However a fixed orifice plate would control the pump, keeping it in its OR regardless of open ended pipe or not, would you not agree?

The option you suggest which will work, uses the valve and I am presuming its a gate valve in a way it has not been designed for or intended to be use.

Yes, A fixed orifice plate will do the same.

but then, here also, the OP does not say what is the situation ahead of the pump, or what type of valve he has installed.

in any case, I am not giving any solution but only suggesting that there are possible issues at hand that might be interesting for the OP to know about, and maybe they are at the base of his enquiry (?)

Very true... we can but offer words of wisdom.... and hope they help!

Not so fast.

I worked in the wet chemical process industry for some years. We built hundreds of process lines, each line having as many as 50 pumps of various sizes and we NEVER restricted the outlet of any of them. Never soft started them either. Never had a problem.

Not so sure that I can buy into your cavitation theory, either if NPSH is adequate and the pump inlet is unrestricted.

Many water supply companies around the UK do this as a matter of routine. The thinking, rightly or wrongly, is that opening the discharge valve slowly after starting the pump leads to less pressure swings in the main downstream, reducing the risks of bursting the downstream main.

Whatever the veracity of the claim, the concern is understandable.

I hear what you say and agree ,Cavitation actually occurs in the impella of the pump,it creates a sound like stones rattling around inside the housing ,if this happens too often it actually creates small holes in the impella ,what is happening is the pump is actually pumping the air out of the water.This can happen without the saftey settings of the motor activating.I have seen impellas that look like they have been sandblasted in the one spot for too long ,this creates a weak spot in the impella which can cause component destruction. Farmer---Jack of all trades but master of none.

Do not wait beyond the 90seconds. In fact you can start opening at 30 seconds into your "Soft starting to full volts in 90 seconds "-- because that giant gate valve will itself take time to full opening -- full pumping flowrate.

Dear Mr. magwenjere,

From your posting I understand it should be a centrifugal pump.

If so, the moment the motor attains it s rated speed, immediately, but slowly, you can open the delivery valve.

RAJESWARI.D

I've given a negative GA because the questions was, "what is the maximum time", not "what is the minimum time". The former is about overheating and the latter is about making the start harder (probably not an issue anyway for a centrifuga/axial flow pump).